We have continued our studies on the metabolism of thyrotropin releasing hormone. Our previous studies resulted in the discovery of a new metabolite of thyrotropin releasing hormone. The new compound, histidyl-proline diketopiperazine has been tested for biological activity. We found that this cyclic dipeptide is substantially more potent than thyrotropin releasing hormone in reducing ethanol-induced sleep in rats. These data indicate that the metabolism of thyrotropin releasing hormone plays a part in its biological effects.